There has been a marked trend toward development in secondary batteries of high capacity (Wh) to be used as the power source of hybrid or pure electric motorcars. Noticeable among them are lithium ion secondary batteries of square type with a high energy density (Wh/kg).
The lithium ion secondary battery of a square type has a generating element including flat wound electrodes formed by winding a positive electrode consisting of a positive electrode foil and a positive electrode active material coated thereon, a negative electrode consisting of a negative electrode foil and a negative electrode active material coated thereon, and a separator for insulation between them one over another in multiple layers. The wound electrodes are electrically connected to the positive and negative electrode terminals which are formed on the lid of the battery container. Moreover, the wound electrodes are held in the battery can of the battery container, with the battery can having its opening sealed by the battery lid welded thereto. The square secondary battery is completed by filling with an electrolyte poured through the inlet of the battery container holding the wound electrodes and subsequently by closing the inlet with a stopper inserted therein and sealing the inlet by means of laser welding.
A multiplicity of square secondary batteries are formed into a battery pack by electrically connecting the positive and negative electrodes of individual units through conducting members such as bus bars. The bus bar is screwed with bolts and nuts or welded to the external terminals of the square secondary battery.
Patent Document 1 discloses a secondary battery having external terminals capable of connection to the bus bar with bolts and nuts. The secondary battery disclosed in Patent Document 1 includes a current collector connected to the generating element, a rivet terminal to electrically connect the current collector to the terminal base arranged on the battery lid, and a terminal base having formed therein a staking hole into which the rivet terminal is inserted and a through hole into which a bolt is inserted. The secondary battery disclosed in Patent Document 1 is characterized in that the rivet terminal is connected and fastened to the terminal base by staking and the rivet terminal is also connected and fastened to the current collector joint by staking and welding.